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Simrad SN90 Reference Manual
A
Turbulent flow
B
Laminar flow
C
Air bubbles
Air bubbles absorb and reflect the sound energy, and they may in worst cases block the
sound transmission altogether.
The boundary layer increases in thickness when it becomes turbulent. The boundary
layer is thin in the forward part of the vessel hull, and increases as it moves aft. The
thickness depends on ships speed and on the roughness of the hull. All objects sticking
out from the hull, or dents in the hull, will disturb the flow and will increase the thickness
of the boundary layer. When the flow speed is high, the turbulence can be violent enough
to destroy the integrity of the water. Small voids or cavities in the water will occur and
this is called cavitation.
Rattle noise
Rattle noise may be caused by loose objects in the vicinity of the transducer, like fixing
bolts. The rattle may also come from loose objects inside the hull.
Interference
Interference from other hydroacoustic equipment on board the same vessel may be an
annoying source of disturbance. Unless the same frequency is used for more than one
piece of equipment only the transmitted pulse will contribute to the interference.
In physics, interference is the phenomenon in which two waves superpose each
other to form a resultant wave of greater or lower amplitude. Interference usually
refers to the interaction of waves that are correlated or coherent with each other,
either because they come from the same source or because they have the same or
nearly the same frequency. Interference effects can be observed with all types of
waves, for example, light, radio, acoustic, surface water waves or matter waves.
https://en.wikipedia.org/wiki/Wave_interference — April 2016
Related topics
Improving the recognition of fish and schools using receiver filters, page 118
Concept descriptions, page 625
Acoustic noise, page 636
640
381294/E
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